[Amps] Fw: Parts needed for Drake L-7 amp

[Michael Tope]

Torrey,

You should be able to remove C47 without any detrimental
effects. I had one fail in my L7. When I removed the capacitor
and then swept plate choke with the MFJ-269, the only
resonance I saw was at 19.1 MHz. This seems to be far
enough away from 17 meters such that it is not a problem
there (the amplifier worked fine on 17 meters after I
removed the capacitor). I don't know how big the error
bars are on this resonance (it may vary significantly
from choke to choke), so it might be worth your while
to sweep the choke with an impedance analyzer to
make sure the resonance doesn't end up too close
to 17 meters. BTW, with the capacitor installed, there
was a self-resonance in my plate choke assembly
at 5.1 MHz.

As far as the 0.82 ohm resistor goes, as mentioned
before, replace it with a suitable glitch resistor. The
circuit breakers on the input side of the L-7 power
supply will take the unit off line if there is a high voltage
fault. You do need a glitch resistor (10 to 30 ohms),
however, to limit the peak fault current to a safe
value during the short period before the circuit
breaker trips and the filter capacitors are discharging.
Globar's carborundum resistors are probably the
best choice, but are kinda of hard to find (Kanthal
doesn't stock them). Ameritron sells 10 ohm x 13
watt x 400 joule pulse rated resistors which are rated
for 1500 volts (they are stamped "RCD175P"). You
can put two or three in series to make a suitable
glitch resistor for the L-7. Just call up the Ameritron
service department. They were cheap.

73 de Mike, W4EF..........................................

----- Original Message -----
From: "Torrey L. Mitchell" <torreym at mchsi.com>
To: <amps at contesting.com>
Sent: Wednesday, November 19, 2003 7:14 PM
Subject: [Amps] Fw: Parts needed for Drake L-7 amp


> Hi all,
>
> Recently I blew up my L-7 by tuning up to max in the SSB position.  I'm
having difficulty finding two components which need replacing.
>
> The first is the green Drake-made capacitor C41 47 pF between the top and
the middle of the HV plate choke.  Mine arced through and doesn't seem
repairable.  Since this is a "home-brew" component made by Drake (to save $$
?) I cannot rush out and buy one.  I'd like to stick to original components
if at all possible.
>
> The second component is the R12 0.82 ohm 2 watt power supply fuse
resistor.  Harbaugh Electronics is the usual reliable source for these.
However, he just moved and won't be open for business again until
mid-December.
>
> Anyone who can help me get my hands on these components?  It would be
immensely appreciated!
>
> 73 de Torrey N9AUJ
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